Mechanical actuators are used in a variety of applications for operating devices such as valves, dampers, doors, etc. Such actuators are used in applications requiring a high level of either torque or linear force. Actuators may be designed to provide their output in either form, which can then be converted to the other by a number of different mechanisms such as a crank arm or rack and pinion. Internally, these actuators typically include a small electric motor driving a reduction gear train for providing the high torque or force output necessary. Typical reduction gear train ratios may be on the order of 1000:1. It is frequently required to limit or control output torque or force, and one way that this is accomplished is by placing a torque limiting or overriding slip clutch at the motor output shaft. When a load requires more torque or force than the design value of such a clutch, the clutch simply slips. In many cases it is important to limit force applied to the controlled device to prevent damage to it.
One type of slip clutch that is often used in low torque situations as a torque-limiting coupling between first and second coaxial shafts such as a motor shaft and the gear train input shaft of an actuator, is the so-called magnetic hysteresis clutch. Such a clutch has a cylindrical armature formed of material with high magnetic remanence and in which an alternating north-south magnetic pattern is permanently formed around its periphery. The armature is mounted for rotation on a first shaft. A cup which is mounted for rotation on a second shaft coaxial with the first shaft, closely fits around the armature's periphery. A special magnetic hysteresis layer is present on the interior cylindrical surface of the cup. As the magnet rotates, it creates a magnetic field in the hysteresis layer which opposes that of the armature. The opposing magnetic fields transfer torque in either direction through the clutch. By properly selecting the strength of the armature's magnetic field and the physical dimensions of the cup and armature, the maximum torque which the clutch can transfer can be controlled relatively accurately. In the actuator application, the torque is transferred by the clutch from the motor to the input gear of the gear train.
A further requirement in some actuator designs is the ability to return the controlled device to a preselected position when an electric power outage occurs. For example, if the controlled device is a fuel valve, when power is lost the valve must be immediately closed to prevent escape of fuel in an uncontrolled manner. One common means for this power out return function is a strong coil spring which is wound or kept wound when the output element moves away from the preselected position, and then is released when a power outage occurs to provide an alternative source of torque for the gear train. The spring-generated torque is then applied to the gear train to return the actuator output element to the preselected position.
Certain types of motors often used in these actuators have cogging torque which resists torque applied to the motor shaft from an external source. Where a coil spring is used for power out return torque for an actuator using such a motor, it is necessary to disconnect the motor from the gear train during spring-powered return. If coupled to the input of the gear train during return operation, such a motor provides resistive torque to the input shaft of the gear train which prevents a spring from returning the output element to its preselected position. Even if the motor does not have cogging torque, its position at the input shaft of the gear train will provide sufficient mechanical drag to require a much larger spring than would otherwise be necessary.
Accordingly, it is necessary in some designs to provide a means of disconnecting the drive motors from the gear train input shafts of the actuators of which they form a part. There are now various types of power operated clutches which can perform this function. However, these devices are relatively complex and expensive, so an inexpensive and simple type of disconnect feature for the motor shaft from the gear train in an actuator would be advantageous.